The present invention relates to a method for event-related determination of process variables in control assemblies, in particular microcomputer-aided control assemblies, in which process variable profiles are detected by cyclic sampling, and processed in a control program. Such methods are generally well-known and used in control assemblies for machines of all types.
Microcomputer-aided control assemblies in which process variables such as distance, pressure, temperature etc are processed are used for controlling machines, handling devices and system parts. In general, assemblies such as these operate cyclically, in a timed mode. In this case, it is necessary to record the process variables as they exist at the sampling time, for which purpose the assemblies have parts which contain memories in which the instantaneous values of the process variables are stored at the sampling time either directly by the clock or in the context of an interrupt service routine. The control program then refers to the instantaneous values for open and closed-loop control purposes in the time period between two samples.
For certain control tasks, it is necessary to detect instantaneous values of process variables at the time of a process event, for example the position value of a machine shaft at the switching time of a measurement probe. Parts containing additional memory are provided for this purpose, in which the instantaneous values of the process variables are stored with respect to a signal which indicates the process event. These parts containing additional memory are required for each process variable and each process event.
Control engineering concepts in which the components of a system are arranged in a decentralized manner and are coupled to one another hierarchically via a bus system are being used increasingly. In this case, the process variables are detected using different components, which are synchronized by suitable mechanisms beyond the bus. However, such mechanisms are not suitable for transmitting the switching signal of a process event precisely at the event time through the bus to the components which require the switching signal. The switching signal therefore has to be passed to these components via an additional connecting line.
It is also possible for a switching signal which indicates the process event to be supplied to a central memory, which stores the actual time of the process event, with, this time being related absolutely or relatively to the sampling times (of which the control program is aware) and with the control program using the instantaneous values of the process variables of the samples on either side of the time of the process event to calculate the magnitude of the process variable corresponding to the time of said process event. However, this solution has the disadvantage that the calculated value has an error from the actual value from the actual profile of the process variable between the sampling times, at the time of the process event. The magnitude of this discrepancy differs, depending on the time interval between the samples and the relationship with the process variable dynamics. The object of the present invention is to provide the calculated value of the process variable with high accuracy.
According to the present invention, this object is achieved by a method in which the time t_(x) of a process event is related absolutely or relatively to the preceding or subsequent cyclic sampling time t_(nxe2x88x921) or t_(n) and the control program determines the instantaneous value, which corresponds to the time t_(x) of the process event, of a process variable p_(x). This determination is made by a calculation from the instantaneous values p_(nxe2x88x921) or p_(n) of the process variable from the samples on either side of the process event, and in that the variable state of the first and second derivations of the process variable p are taken into account when determining the process variable p_(x) for the time t_(x) of the process event. The application for the present invention as originally filed in German is incorporated herein by reference.
A preferred embodiment of the present invention is obtained by taking account of the third derivative of the process variable p when determining the variable state. The method can be carried out easily in that the process variable p_(x) for the time t_(x) of the process event is determined with reference to the value of the variable state p which results from the closest sample t_(nxe2x88x921) or t_(n) to the time t_(x) of the process event.
However, as an alternative, it is also possible in the method as initially disclosed for the time t_(x) of a process event to be related absolutely or relatively to the preceding or subsequent cyclic sampling time t_(nxe2x88x921) or t_(n), and wherein the control program determines the instantaneous value, which corresponds to the time of the process event, of a process variable p_(x) by calculation via a ratio assessment. This calculation takes into account the relative position of the time of the process event t_(x), from the instantaneous values p(nxe2x88x921) or p_(n) of the process variable at the time of the samples on either side of the process event. There is therefore no need either for any parts containing additional memories in the control assemblies or for any additional connecting lines for the switching signal relating to the process event between components arranged in a decentralized manner. Furthermore, since the process variable p is detected in components which are coupled by means of a bus system, decentralized multi-component operation is feasible.